6lo M. Sethi, Ed.
Internet-Draft Ericsson
Updates: 6775 (if approved) P. Thubert
Intended status: Standards Track Cisco
Expires: February 23, 2017 B. Sarikaya, Ed.
Huawei USA
August 22, 2016
Address Protected Neighbor Discovery for Low-power and Lossy Networksdraft-sarikaya-6lo-ap-nd-04
Abstract
This document defines an extension to 6LoWPAN Neighbor Discovery.
This extension is designed for low-power and lossy network
environments and it supports multi-hop operation. Nodes supporting
this extension compute a Cryptographically Unique Interface ID and
associate it with one or more of their Registered Addresses. The
Cryptographic ID (Crypto-ID) uniquely identifies the owner of the
Registered Address. It is used in place of the EUI-64 address that
is specified in RFC 6775. Once an address is registered with a
Cryptographic ID, only the owner of that ID can modify the state
information of the Registered Address in the 6LR and 6LBR.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 23, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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any device claiming the same EUI-64 interface ID may take over an
existing registration and attract the traffic for that address. The
address registration mechanism in [RFC6775] is limited as it does not
require a node to prove its ownership of the EUI-64 Interface ID.
Therefore, any node connected to the subnet and aware of the
registered address to EUI-64 interface ID mapping may effectively
fake the same interface ID and steal an address.
In this document, we extend 6LoWPAN ND to protect the address
ownership with cryptographic material, but as opposed to Secure
Neighbor Discovery (SEND) [RFC3971] and Cryptographically Generated
Addresses (CGAs) [RFC3972], the cryptographic material generated is
not embedded in the Interface ID (IID) as an IPv6 address. Instead,
the generated cryptographic ID is used as a correlator associated
with the registration of the IP address. This approach is made
possible with 6LoWPAN ND [RFC6775], where the 6LR and the 6LBR
maintain state information for each Registered Address. If a
cryptographic ID is associated with the first 6LoWPAN ND
registration, then it can be used to validate any future updates to
the registration.
In order to achieve this ownership verification, in this extension
specification, the EUI-64 interface ID used in 6LoWPAN ND is replaced
with cryptographic material whose ownership can be verified. The
extension also provides new means for the 6LR to validate ownership
of the registration, and thus, the ownership of registered address.
The resulting protocol is called Protected Address Registration
protocol (ND-PAR).
In ND-PAR, a node typically generates one 64-bit cryptographic ID
(Crypto-ID) and uses it as Unique Interface ID in the registration of
one (or more) of its addresses with the 6LR, which it attaches to and
uses as default router. The 6LR validates ownership of the
cryptographic ID typically upon creation or update of a registration
state, for instance following an apparent movement from one point of
attachment to another. The ARO option is modified to carry the
Unique Interface ID, and through the DAR/DAC exchange.
Compared with SeND, this specification saves ~1Kbyte in every NS/NA
message. Also SeND requires one cryptographic address per IPv6
address. This specification separates the cryptographic identifier
from the IPv6 address so that a node can have more than one IPv6
address protected by the same cryptographic identifier. SeND forces
the IPv6 address to be cryptographic since it integrates the CGA as
an IID. 6LoWPAN derives the IPv6 address from other things like a
short address in 802.15.4 to enable a better compression.
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Internet-Draft Address Protection ND for LLN August 20162. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Readers are expected to be familiar with all the terms and concepts
that are discussed in [RFC3971], [RFC3972], [RFC4861], [RFC4919],
[RFC6775], and [I-D.ietf-6lo-backbone-router] which proposes an
evolution of [RFC6775] for wider applicability.
This document defines Crypto-ID as an identifier of variable size
which in most cases is 64 bits long. It is generated using
cryptographic means explained later in this document.
The document also conforms to the terms and models described in
[RFC5889] and uses the vocabulary and the concepts defined in
[RFC4291] for the IPv6 Architecture.
This document uses [RFC7102] for Terminology in Low power And Lossy
Networks.
3. Requirements
In this section we state requirements of a secure neighbor discovery
protocol for low-power and lossy networks.
o The protocol MUST be based on the Neighbor Discovery Optimization
for Low-power and Lossy Networks protocol defined in [RFC6775].
RFC6775 utilizes optimizations such as host-initiated interactions
for sleeping resource-constrained hosts and elimination of
multicast address resolution.
o New options to be added to Neighbor Solicitation messages MUST
lead to small packet sizes, especially compared with existing
protocols such as SEcure Neighbor Discovery (SEND). Smaller
packet sizes facilitate low-power transmission by resource-
constrained nodes on lossy links.
o The support for this registration mechanism SHOULD be extensible
to more LLN links than IEEE 802.15.4 only. Support for at least
the LLN links for which a 6lo "IPv6 over foo" specification
exists, as well as Low-Power Wi-Fi SHOULD be possible.
o As part of this extension, a mechanism to compute a unique
Identifier should be provided with the capability to form a Link
Local Address that SHOULD be unique at least within the LLN
connected to a 6LBR.
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o The Address Registration Option used in the ND registration SHOULD
be extended to carry the relevant forms of Unique Interface
IDentifier.
o The Neighbour Discovery should specify the formation of a site-
local address that follows the security recommendations from
[RFC7217].
4. Protocol Interactions
Protected address and registration neighbor discovery protocol (ND-
PAR) modifies Neighbor Discovery Optimization for Low-power and Lossy
Networks [RFC6775] as explained in this section.
4.1. Overview
The scope of the present work is a 6LoWPAN Low Power Lossy Network
(LLN), typically a stub network connected to a larger IP network via
a Border Router called a 6LBR per [RFC6775].
---+-------- ............
| External Network
|
+-----+
| | LLN Border
| | router
+-----+
o o o
o o o o
o o LLN o o o
o o o o
o
Figure 1: Basic Configuration
The 6LBR maintains a registration state for all devices in the
attached LLN, and, in conjunction with the first-hop router (the
6LR), is in a position to validate uniqueness and grant ownership of
an IPv6 address before it can be used in the LLN. This is a
fundamental difference with a classical network that relies on IPv6
address auto-configuration [RFC4862], where there is no guarantee of
ownership from the network, and any IPv6 Neighbor Discovery packet
must be individually secured [RFC3971].
In a mesh network, the 6LR is directly connected to the host device.
This specification expects that the peer-wise layer-2 security is
deployed so that all the packets from a particular host are securely
identifiable by the 6LR. The 6LR may be multiple hops away from the
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6LBR. Packets are routed between the 6LR and the 6LBR via other
6LRs. This specification expects that a chain of trust is
established so that a packet that was validated by the first 6LR can
be safely routed by the next 6LRs to the 6LBR.
[I-D.ietf-6tisch-architecture] suggests to use of RPL [RFC6550] as
the routing protocol between the 6LRs and the 6LBR, and leveraging a
backbone router [I-D.ietf-6lo-backbone-router] to extend the LLN in a
larger multilink subnet [RFC4903]. In that model, a registration
flow happens as shown in Figure 2. Note that network beyond the 6LBR
is out of scope for this document.
6LoWPAN Node 6LR 6LBR
(RPL leaf) (router) (root)
| | |
| 6LoWPAN ND |6LoWPAN ND+RPL | Efficient ND
| LLN link |Route-Over mesh| IPv6 link
| | |
| NS(ARO) | |
|-------------->| |
| 6LoWPAN ND | DAR (then DAO)|
| |-------------->|
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | DAC |
| |<--------------|
| NA(ARO) | |
|<--------------| |
Figure 2: (Re-)Registration Flow over Multi-Link Subnet
A new device that joins the network auto-configures an address and
performs an initial registration to an on-link 6LR with an NS message
that carries a new Address Registration Option (ARO) [RFC6775]. The
6LR validates the address with the central 6LBR using a DAR/DAC
exchange, and the 6LR confirms (or denies) the address ownership with
an NA message that also carries an Address Registration Option.
The registration mechanism in [RFC6775] was created for the original
purpose of Duplicate Address Detection (DAD), whereby use of an
address would be granted as long as the address is not already
present in the subnet. But [RFC6775] does not require that the 6LR
use the registration for source address validation (SAVI) [RFC7039].
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Protected address registration protocol proposed in this document
enforces SAVI. With this we ensure that only the correct owner uses
the registered address in the source address field. Therefore a
destination node can trust that the source is the real owner without
using SeND. All packets destined for a node go through the 6LR to
which it is attached. The 6LR maintains state information for the
registered addressed along with the MAC address, and link-layer
cryptographic key associated with that node. The 6LR therefore only
delivers packets to the real owner based on its state information.
In order to validate address ownership, the registration mechanism
(that goes all the way to the 6LBR with the DAR/DAC) enables the 6LBR
to correlate further claims for a registered address from the device
to which it is granted, based on a Unique Interface IDentifier (UID).
This UID is derived from the MAC address of the device (EUI-64).
This document uses a randomly generated value as an alternate UID for
the registration. Proof of ownership of the UID is passed with the
first registration to a given 6LR, and enforced at the 6LR, which
validates the proof. With this new operation, the 6LR allows only
packets from a connected host if the connected host owns the
registration of the source address of the packet.
In a multihop 6LoWPAN, the registration with Crypto-ID is propagated
to 6LBR as described in Section 4.3. If a chain of trust is present
between the 6LR and the 6LBR, then there is no need to propagate the
proof of ownership to the 6LBR. All the 6LBR needs to know is that
this particular UID is randomly generated, so as to enforce that any
update via a different 6LR is also random.
4.2. Updating RFC 6775
Protocol interactions are as defined in Figure 2. The Crypto-ID is
calculated as described in Section 4.2.1.
The Target Address field in NS message is set to the prefix
concatenated with the node's address. This address does not need
duplicate address detection as Crypto-ID is globally unique. So a
host cannot steal an address that is already registered unless it has
the key used for generating the Crypto-ID. The same Crypto-ID can
thus be used to protect multiple addresses e.g. when the node
receives a different prefix.
Local or on-link protocol interactions are shown in Figure 3.
Crypto-ID and ARO are passed to and stored by the 6LR/6LBR on the
first NS and not sent again in the next NS. The operation starts
with 6LR sending a Router Advertisement (RA) message to 6LN.
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The 6LR/6LBR ensures first-come/first-serve by storing the ARO and
the Crypto-ID correlated to the node being registered. The node is
free to claim any address it likes as long as it is the first to make
such a claim. The node becomes owner of that address and the address
is bound to the Crypto-ID in the 6LR/6LBR registry. This procedure
avoids the constrained device to compute multiple keys for multiple
addresses. The registration process allows the node to tie all the
addresses to the same Crypto-ID and have the 6LR/6LBR enforce first-
come first-serve after that.
A condition where a 6LN uses multiple IPv6 addresses may happen when
the node moves at a different place and receives a different prefix.
In this scenario, the node uses the same Crypto-ID to protect its new
IPv6 address. This prevents other nodes from stealing the address
and trying to use it as their source address.
Note that if the device that moves always forms new MAC and IP
address [RFC6775], then this new address can be used for
registration. In case of a collision of the new MAC and therefore IP
address, the node can easily form a new IPv6 address. This is one
case where the use of Crypto-ID would not be needed. Crypto-ID or
ND-PAR should be activated when the IP address is claimed at another
place, or for a different MAC address at the same place, e.g. for MAC
address privacy [I-D.ietf-6man-ipv6-address-generation-privacy].
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6LN 6LR
| |
|<------------------- RA --------------------------|
| |
|----------- NS with ARO and Crypto-ID ----------->|
| |
|<---------- NA with ARO (status=req-proof) -------|
| |
|----------- NS with ARO and Crypto-ID ----------->|
| |
|<---------------- NA with ARO --------------------|
| |
... ...
| |
|------------ NS with ARO and Crypto-ID ---------->|
| |
| |
|<---------------- NA with ARO --------------------|
... ...
| |
|----------- NS with ARO and Crypto-ID ----------->|
| |
| |
|<---------------- NA with ARO --------------------|
Figure 3: On-link Protocol Operation
Elliptic Curve Cryptography (ECC) is used in the calculation of
cryptographic identifier (Crypto-ID). The digital signature is
constructed by using the 6LN's private key over its EUI-64 (MAC)
address. The signature value is computed using the ECDSA signature
algorithm and the hash function used is SHA-256 [RFC6234]. Public
Key is the most important parameter in CGA Parameters (sent by 6LN in
an NS message). ECC Public Key could be in uncompressed form or in
compressed form where the first octet of the OCTET STRING is 0x04 and
0x02 or 0x03, respectively. Point compression can further reduce the
key size by about 32 octets.
After calculating its Crypto-ID, a 6LN sends it along with the CGA
parameters in the first NS message, see Figure 3. In order to send
Crypto-ID, a modified address registration option called Enhanced
Address Registration Option (EARO) is defined in Figure 4. As
defined in the figure this ID is variable length, varying between 64
to 128 bits. This ID is 128 bits long only if it is used as IPv6
address. This may happen when some application uses one IP address
of the device as device ID. It would make sense in that case to
build a real CGA IPv6 address. The prefix of the address would be
obtained from prefix information option (PIO in RA) [RFC4861].
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6LN also sends some other parameters to enable 6LR or 6LBR to verify
the Crypto-ID. The option shown in Figure 5 can be used. In the
figure, CGA Parameters field contains the public key, prefix and some
other values. It is a simplified form of CGA Option defined in
[RFC3971].
4.2.1. Crypto-ID Calculation
First, the modifier is set to a random or pseudo-random 128-bit
value. Next, concatenate from left to right the modifier, 9 zero
octets and the ECC public key. SHA-256 algorithm is applied on the
concatenation. The 112 leftmost bits of the hash value is taken.
Concatenate from left to right the modifier value, the subnet prefix
and the encoded public key. NIST P-256 is executed on the
concatenation. The leftmost bits of the result is used as the
Crypto-ID. The length is normally 64 bits, however it could be 128
bits.
In respecting the cryptographical algorithm agility [RFC7696], Curve
25519 [RFC7748] can also be used instead of NIST P-256. This is
indicated by 6LN by setting the Crypto Type field in CGA Parameters
Option to a value of 1. If 6LBR does not support Curve 25519, it
will set Crypto Type field to zero. This means that the default
algorithm (NIST P-256) will be used.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |C|T| TID | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Owner Unique ID (EUI-64 or equivalent) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Enhanced Address Registration Option
Type:
TBA1
Length:
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8-bit unsigned integer. The length of the option (including the
type and length fields) in units of 8 bytes. The value 0 is
invalid. A value of 3 with the C flag set indicates a Crypto-ID
of 128 bits.
Status:
8-bit unsigned integer. Indicates the status of a registration in
the NA response. MUST be set to 0 in NS messages. See below.
Reserved:
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
C:
C bit when set is used to indicate that Owner Unique ID fields
contains Crypto-ID.
T and TID:
Defined in [I-D.ietf-6lo-backbone-router].
Owner Unique ID:
In this specification, this field contains Crypto-ID, a variable
length field to carry the Crypto-ID or random UID. This field is
normally 64 bits long. It could be 128 bits long if IPv6 address
is used as the Crypto-ID.
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The type of cryptographic algorithm used in calculation Crypto-ID.
Default value of all zeros indicate NIST P-256. A value of 1 is
assigned for Curve 25519. New values may be defined later.
Modifier:
128 bit random value.
Subnet Prefix:
64 bit subnet prefix.
Public Key:
ECC public key of 6LN.
Padding:
A variable-length field making the option length a multiple of 8,
containing as many octets as specified in the Pad Length field.
4.3. Multihop Operation
In multihop 6LoWPAN, 6LBR sends RAs with prefixes downstream and it
is the 6LR that receives and relays them to the nodes. 6LR and 6LBR
communicate with the ICMPv6 Duplicate Address Request (DAR) and the
Duplicate Address Confirmation (DAC) messages. The DAR and DAC use
the same message format as NS and NA with different ICMPv6 type
values.
In ND-PAR we extend DAR/DAC messages to carry cryptographically
generated UID. In a multihop 6LoWPAN, the node exchanges the
messages shown in Figure 2. The 6LBR must be aware of who owns an
address (EUI-64) to defend the first node if there is an attacker on
another 6LR. Because of this the content that the source signs and
the signature needs to be propagated to the 6LBR in DAR message. For
this purpose the DAR message sent by 6LR to 6LBR MUST contain CGA
Parameters and Digital Signature Option carrying the CGA that the
node calculates and its public key. DAR message also contains ARO.
It is possible that occasionally, 6LR may miss the node's UID (that
it received in ARO). 6LR should be able to ask for it again. This is
done by restarting the exchanges shown in Figure 3. The result
enables 6LR to refresh the information that was lost. 6LR MUST send
DAR message with ARO to 6LBR. 6LBR as a reply forms a DAC message
with the information copied from the DAR and the Status field is set
to zero. With this exchange, the 6LBR can (re)validate and store the
information to make sure that the 6LR is not a fake.
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In some cases 6LBR may use DAC message to signal to 6LR that it
expects Crypto-ID from 6LR also asks 6LR to verify the EUI-64 6LR
received from 6LN. This may happen when a 6LN node is compromised
and a fake node is sending the Crypto-ID as if it is the node's EUI-
64. Note that the detection in this case can only be done by 6LBR
not by 6LR.
5. Security Considerations
The same considerations regarding the threats to the Local Link
Network covered in [RFC3971] apply.
The threats discussed in Section 9.2 of [RFC3971] are countered by
the protocol described in this document as well.
Collisions of Crypto-ID is a possibility that needs to be considered.
The formula for calculating probability of a collision is 1 -
e^{-k^2/(2n)}. If the Crypto-ID is 64-bit long, then the chance of
finding a collision is 0.01% when the network contains 66 million
nodes. It is important to note that the collision is only relevant
when this happens within one stub network (6LBR). A collision of ID
in ND-PAR is a rare event. However, when such a collision does
happen, the protocol operation is not affected, although it opens a
window for a node to hijack an address from another. The link-layer
security ensures that the nodes would normally not be aware of a
collision on the subnet. If a malicious node is able to gain
knowledge of a collision through other means, the only thing that it
could do is to steal addresses from the other honest node. This
would be no different from what is already possible in a 6lo network
today.
6. IANA considerations
IANA is requested to assign two new option type values, TBA1 and TBA2
under the subregistry "IPv6 Neighbor Discovery Option Formats".
7. Acknowledgements
We are grateful to Rene Struik and Robert Moskowitz for their
comments that lead to many improvements to this document.
8. References8.1. Normative ReferencesSethi, et al. Expires February 23, 2017 [Page 14]